Late Jurassic and Early Cretaceous Muddy Coastline and Estuarine Bay Deposition in West Central Alberta

Abstract

Around Edson, Alta., subsurface strata above the Late Jurassic ‘Upper Fernie Shale’ are commonly assigned to the Cretaceous Ellerslie Mbr. (Aptian, Mannville Grp.). Typically the strata are interpreted as fluvial valley-fills that accumulated during/following the cutting of the ‘Sub-Cretaceous Unconformity’. Our core examinations from this area instead yielded no apparent evidence for prolonged subaerial exposure at the top of the ‘Upper Fernie Shale’. To assess stratigraphy and deposition of strata between the Niton Mbr. (Fernie Formation) and the Calcareous Mbr. (Lower Mannville Fm.) the interval was analyzed sedimentologically and petrologically. Cores from 75 wells between townships and ranges 11-051W5 and 16-056W5 were examined and palynological age dates were obtained. The ‘Upper Fernie Shale’ comprises graded, micro-laminated mudstones and minor sandstones that accumulated during a Kimmeridgian transgression. Gutter casts, combined-flow and hummocky cross-stratifications indicate wave and storm reworking between SWWB and FWWB. High mud-/low sandstone volumes are suggestive of active deposition from suspended sediments in a muddy coastal system. Locally roots, spherulitic siderite and clay alignment fabrics agree with freshwater-influenced wetland formation following stabilization of muddy coastal banks. Upper Fernie shales are overlain by wavy/lenticular sandstones and shales of Kimmeridgian/Tithonian age. Dinoflagellates, minor bioturbation, rhythmic bedding, flasers, and bidirectional ripples are indicative of tidal influence in a brackish bay. This ‘ribbon strata’ may grade laterally into m-thick quartzose sandstones. Absence at the top of the ‘Upper Fernie Shale’ of paleosols and a detectable time gap excludes a major hiatus at this stratigraphic level. Instead, ribbon strata extend upward into the Valanginian, where they terminate against a mature, regional paleosol. Our findings indicate that the stratigraphic framework for West-Central Alberta requires revision. Strata above the ‘Upper Fernie Shale’ are older and lithologically distinct from the Ellerslie Mbr. in its type area. The evidence we collected points to an aerially extensive, muddy marine-paralic system in the area from the Late Jurassic into the Early Cretaceous. Thus, the Jurassic-Cretaceous boundary is located within a conformable succession. Importantly the ‘Sub-Cretaceous Unconformity’ is instead a ‘mezzanine’ unconformity as it is in reality an ‘intra-Cretaceous’ unconformity.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016